Two Kinds of CMEs

Science Nugget: 25 May 2001

What are the two kinds?

It has been known for long time (see
MacQueen and Fisher (1983)) that
there are two kinds of CMEs in terms of speeds. CMEs belonging to the first category,
possibly flare-associated, have high and constant speeds, and those in the second
category, possibly associated with filament eruptions, have low speeds but
acceleration profiles. This classification has recently been extended to data from
SOHO/LASCO, which has significantly higher sensitivity than previous instruments,
making it possible to observe diffuse "halo" CMEs. The reference is
Sheeley et al., JGR, 1-4, 24739 (1999) (sorry no abstract
of paper available at ADS, maybe AGU's policy?). They primarily dealt with halo
CMEs using LASCO C2 and C3 data. They found that fast ones, which tend to have
ragged structures, decelerate rather quickly (typically 1000 --> 500 km/s in less
than an hour), and that slow ones, which tend to have smooth structures, get accelerated
to constant speeds sooner than their non-halo counterparts (i.e., those that originate
near the limb.)

I would like to know if the correlation between flares and fast CMEs is seen for
halo events. In the following tables
(separately for slow events and
fast events),
I replicate the information from the Sheeley et al. paper, but give only typical
speeds. Sheeley et al. measured the CME speeds along different radial paths, and for
different structures, but we show only one piece of information per event. For
slow events, V with suffix "A" stands for the constant speed after the acceleration is
over. For fast events, the two speeds with suffices "0" and "1" indicate how the
initial high speed decreases (within a half to a few hours).

I inserted the time of the first CME signature in C2 data, after looking at all the
movies. I also looked for the CME signatures in all the available SXT and EIT data,
and entered their locations if I found something. The locations of the signatures
may or may not correspond to active regions. I studied GOES light curves to see
if flares occurred around the CME times. Lastly, I checked the NOAA site for SEP
events and elevated KP indices, in likely association with the CMEs. Some people
may say there should be better data of these kinds, but I list these only for basic
information.

What do we see in the tables?

As expected, the events classified as "slow" in the Sheeley et al. paper are
hardly correlated with major flares. To me it is just a matter of semantics to
call a GOES A-class event a flare. Moreover, lower coronal signatures for these
CMEs are mostly found outside active regions. The events labeled as "fast" are
a bit problematic, since their correlation with flares or even active regions
is not very high. But we have to remember LASCO observations do not tell us
whether a CME is moving towards us of away from us. Yes, I think all the ones
without signatures at lower corona occurred on the other side of the Sun, or at
least way behind the limb.

Here are two examples to illustrate this. First, for the 31 Mar 1998 event,
it is clear from the following SXT movie that the backside activity was more
spectacular.

About two weeks before this event, we witnessed a rapidly growing
active region. This can be seen, for example, in
MDI white-light images.
This was AR 8179 (at latitude S20), whose central meridian passage was 15 March 1998,
and application of the usual differential rotation formula shows that it was about
67 degrees behind the east limb. It is likely that the 31 Mar 1998 CME was
associated with a flare from this region, which would have been very intense if
it occurred on the visible disk. In less than a week, we welcomed the return of this
region as AR 8194, which still produced some major events.

One more example is the 11 May 1998 event, which probably occurred in a
similar configuration to the
Khan-Hudson events, i,e., involving large-scale
trans-equatorial loops. Actially it was only two days after their last event. Because
of a larger angle behind the west limb, there was hardly a flare. But the following
sequence of SXT images of the west limb (click to enlarge) shows a dynamical
evolution, which was probably the CME itself.

Note that we are only rarely fortunate enough to see such direct signatures of
large-scale CMEs, because either the cadence is irregular/relatively low or
the field of view is too small.

Concluding remarks

There are a few more fast events from the backside. In the new table
(click here), they are crossed out.
Also, three very fast events were associated with major flares, as they appear
blinking in the table. The last event is shown to be not very fast, but
if a different path is taken, say in the northeastern direction, the speed
may be higher (this represents one difficulty of this kind of analysis on
LASCO data).
We note all of these were impulsive flares, and
as we indicated earlier, intense, impulsive flares,
distinct from long-decay events (which we know are well correlated with
CMEs),
can be associated with fast and extended CMEs. Certain flares are
just not an aftermath of a CME, but may actively participate in
the acceleration of a CME. Of course, it is still reasonable to think
both flares and CMEs may be attributable to a same magnetic instability.

Identification of backside events for the slow CMEs may be less
obvious, since one or two events are still hard to understand in terms of
solar signatures. The best example is the 6 January 1997 event. Can this
possibly be a backside event? One strong reason for this CME being the
front side event is the significant magnetic cloud. But in order to understand
the solar origin of CMEs, geo-effectiveness can be a distracting factor.
In fact, in
a paper by Crooker, it is mentioned that "3 of 4 halo backside CMEs also were
followed within 5 days by storms." See this plot
from the paper.